Metal-assisted chemical etching (MacEtch or MaCE) is a local electrochemical etching method capable of producing anisotropic, high-aspect ratio semiconductor structures with a simple wet etching process. MacEtch can avoid ion-induced damage and sidewall scallops characteristic of conventional deep reactive ion etching. Nanowires, vias and trenches with electronic and optoelectronic device applications have been demonstrated using MacEtch on silicon and compound semiconductor substrates.
A typical silicon MacEtch process starts by depositing a metal catalyst onto a silicon substrate. The metal catalyst can be patterned into any arbitrary two-dimensional shape. Then, the silicon substrate is immersed in a solution mixture of a suitable acid and oxidant to selectively oxidize and dissolve portions of the silicon substrate localized under the metal catalyst. During MacEtch, at the liquid-catalyst interface, electronic holes are produced by the reduction reaction of the oxidant (cathodic reaction) and injected in the valence band to produce oxidized silicon (anodic reaction), which is selectively dissolved by the acid.
Noble metals such as gold (Au) have been reported to be effective MacEtch catalysts. However, despite various advantages and simplicity, gold catalysts may be associated with deep level defects in silicon; thus, there are limitations in applying MacEtch to electronic device applications, specifically front end of the line (FEOL) and back end of the line (BEOL) processes in complementary metal-oxide-semiconductor (CMOS) fabrication. In addition, the MacEtch process can be associated with “inverse” etching, where regions of the semiconductor substrate that are not localized under the metal catalyst undergo unintended etching.